1 Field of the Invention
The invention relates to a packet switching system comprising at least a switching device which comprises
a main memory for buffering packets arriving by auxiliary lines, PA1 a main memory controller for generating addresses to store the packets in the main memory and for controlling the write and read operations of the packets, and PA1 a demultiplexer controlled by the main memory controller, for transporting the packets by trunk lines. PA1 an address memory for producing an address for a write operation, PA1 a respective buffer assigned to each trunk line, for buffering the addresses produced by the address memory, PA1 and a decoding device, which is provided PA1 for receiving the addresses used for the read operation and coming from the buffers, and PA1 after the first issue of all the addresses, for producing further addresses in the order realised by the buffers. PA1 a main memory for buffering cells coming from the station or the controller and arriving by at least a receive ring line, PA1 a main memory controller comprising
2. Discussion of the Related Art
Such a packet switching system is known from the title "ATM--Die Technik des Breitband-ISDN" by Gerd Siegmund, R.v. Deckers' Verlag, Heidelberg, 1993, pages 169 to 176. The switching device described there operates in the asynchronous transfer mode.
When an asynchronous transfer mode is used in a switching system, payload, for example, telephone, video or sound signals, is sent in blocks of a fixed length through arrangements for digital signal processing. By a block of a fixed length is meant a cell which contains a given number of bytes (for example, 53 bytes). Each cell contains a header field having a length of, for example, 5 bytes, and an information field which accommodates the payload and has a length of, for example, 48 bytes. In such a header field are available routing indicators, error recognition data and control data. Routing indicators are meant to be understood as trunk identifiers and link identifiers. The link identifier, also referenced VCI (virtual channel identifier), contains the description of the destination of the cell in the system. To transmit a cell, a virtual channel is made available in response to the virtual channel identifier. As a rule, a VCI is changed each time a switching point is reached. A trunk group of various virtual channels is referenced a virtual path. A virtual path is identified by the trunk identifier. Such a virtual path is called a virtual path identifier (VPI). Cells are assigned to consecutively determined periods of time. The length of such a period of time depends on the clock rate used as a basis of the transfer components. If no payload is available, empty cells i.e. cells without payload, are transmitted in such a period of time. Cells containing payload are referenced payload cells.
Said switching device is based on the main-memory principle. All arriving cells are written in a main memory or read from its output. It is also possible that a cell is copied then. The main memory controller marks the cells in the main memory and controls the write and read operations of the main memory. The circuit structure of the switching device will not be further described in this publication.